Babies take a critical step toward learning to speak before they can say a word or even babble. By 3 months of age, infants flexibly use three types of sounds — squeals, growls and vowel-like utterances — to express a range of emotions, from positive to neutral to negative, researchers say.

Attaching sounds freely to different emotions represents a basic building block of spoken language, say psycholinguist D. Kimbrough Oller of the University of Memphis in Tennessee and his colleagues. Any...